The present invention relates to a digital data processing system in which, in order to protect an analog system such as an electric power system, various parameters such as current or voltage representative of the system are sampled at predetermined time intervals and converted into digital data which is applied to a digital processing device for processing. From the results of the data processing, the conditions of the system are determined and control outputs are provided according to the conditions thus determined to protect the system.
In a conventional on-line control system using a digital processing device, a plurality of sensing units with current transformers (CT) or voltage transformers (PT) are provided to input system data to a central processing unit. FIG. 1 is a block diagram of a conventional system of this type where data simultaneously provided at the sensing units is sampled and converted into digital data and applied to and processed by the digital processing device. A specific application of this system arrangement to an electric power system will be described.
In FIG. 1, reference numerals 1 and 2 designate spaced-apart sensing units in an electric power system which may, for instance, be at installation points of current transformers (CT) or voltage transformers (PT). By way of example, if current is the sensed parameter, the sensing units 1 and 2 will include sensors 11a and 11b, namely, current transformers (CT); analog-to-digital converter units 12a and 12b including analog filters which sample the output analog current outputs of the sensors 11a and 11b at predetermined time intervals to convert them into digital data; sample-and-hold circuits and A/D (analog-to-digital) converters; transmitters 13a and 13b; control circuits 14a and 14b for synchronizing the sampling timing of the two analog-to-digital converter units 12a and 12b; and receivers 15a and 15b which receive master clock signals from an outside source and drive the control circuits 14a and 14b, respectively.
A master clock signal generating circuit 18 including an oscillator and a frequency division circuit is provided at one position in the electric power system to generate a master clock signal which is applied to all of the sensing units to synchronize the sampling timing. Transmitters 17a and 17b are provided to transmit the master clock signal. The sampling signal from the master clock signal generating circuit 8 is applied through the sample signal transmitters 17a and 17b and transmission lines 20 to the sensing units 1 and 2, respectively, where it is received by the sampling signal receivers 15a and 15b and is then applied to the control circuits 14a and 14b, respectively.
The sensors 11a and 11b are provided to detect the designated system parameters. The analog parameters sensed by the sensors 11a and 11b are applied to the analog-to-digital converter units 12a and 12b where they are sampled simultaneously with the sampling signals from the control circuits 14a and 14b and are converted into digital data which is transmitted to a digital processing device 19 through the transmission lines 20 by the data transmitting devices 13a and 13b. The digital data is received by data receiving devices 16a and 16b and then applied to the digital processing device 19 for processing. Thus, the digital processing device receives and transmits the sample values at the same time in the sensing units 1 and 2.
The above-described system is disadvantageous in the following points: Since the sampling signal is supplied to the sensing points from the digital processing device side, the physical arrangement of the system is necessarily large and also the number of components is considerably large which makes the reliability of the system undesirably low.